1. Field of the Invention
The present invention relates to a filter device, and more particularly, to a microwave filter using a TM (transverse magnetic) multiple-mode resonator.
2. Description of the Related Art
In a conventional filter device, an input-output coupling loop 51 located on a metal panel 52, as shown in FIG. 10, has been used in order to electromagnetically couple a TM multiple-mode resonator and an input-output electrode. The metal panel 52 is formed of a metal having a high electrical conductivity such as copper or brass. The coupling loop 51 is also formed of a metal having a high electrical conductivity such as copper or brass, and fixed on the metal panel 52 by soldering or the like. Signals are transmitted to the coupling loop 51 through an inner conductor 502 of a coaxial cable 501. An outer conductor 503 of the coaxial cable 501 is connected to an earth electrode 504 formed on the metal panel 52. The coupling loop 51 may be formed of wire. The metal panel 52 is attached to a dielectric resonator 53 as shown in FIG. 11. The degree of electromagnetic coupling between dielectric columns in the dielectric resonator 53 and the coupling loop 51 is controlled by adjusting the position of the coupling loop 51 (see FIG. 12), or by changing the shape of the coupling loop 51 (see FIG. 13).
However, the above-mentioned prior art has the following problems.
Since the coupling loop 51 is usually fixed on the metal panel 52 by soldering, it is not always easy to change the position thereof. Moreover, when pressure is applied to the coupling loop 51 so as to change the shape thereof, the bonding force of the solder between the coupling loop 51 and the metal panel 52 sometimes weakens.
When a plurality of dielectric resonators 53 are prepared and metal panels 52 are respectively attached thereto, if coupling loops 51 on the metal panels 52 are each formed of wire, it is difficult to equalize the degrees of coupling between the dielectric resonators 53 and the respective coupling loops 51 because it is difficult to give the same shape to all the wires. This results in low productivity in attaching the metal panels 52 to the dielectric resonators 53.
The dielectric resonator 53 comprises two dielectric columns 601 and 602. When the dielectric resonator 53 is operated as a TM multiple-mode resonator, electromagnetic field distributions respectively inherent in the dielectric columns 601 and 602 arise on the peripheries thereof, and the degrees of coupling between the dielectric columns 601 and 602 and the coupling loop 51 vary according to the electromagnetic field distributions. If the shape of the loop 51 is changed in a conventional manner, the degrees of coupling between the dielectric columns 601 and 602 and the coupling loop 51 are both changed thereby, so that it is difficult to separately change the degree of coupling between the loop 51 and the dielectric column 601 and the degree of coupling between the loop 51 and the dielectric column 602.